The present invention relates to a plate exchange apparatus for a printing press, for removing a plate used from a plate cylinder and mounting a plate to be used on the plate cylinder.
In a variety of printing presses, leading- and trailing-side plate lockup devices each consisting of a plate lockup table extending in an axial direction of the plate cylinder and gripper plates pivotally supported on the plate lockup table are arranged in a circumferential gap of a plate cylinder. When a plate is to be mounted on the plate cylinder, the plate, one end of which is gripped by the plate lockup table and the gripper plates of the leading-side plate lockup device, is wound around the circumferential surface of the plate cylinder upon almost one revolution of the plate cylinder. The other end of the plate is gripped by the plate lockup table and the gripper plates of the trailing-side plate lockup device and is mounted on the plate cylinder. When the plate is to be removed from the plate cylinder, the plate is released from the trailing-side plate lockup device and is rewound from the circumferential surface of the plate cylinder upon almost one revolution of the plate cylinder. The plate is finally released from the leading-side plate lockup device. The removed plate is removed outside the printing press.
When the specifications of printed matters are changed, an old plate used is exchanged with a new plate, as described above. However, this manual plate exchange operation requires much labor, is time-consuming, and is dangerous. In recent years, a full-automatic plate exchange apparatus for mechanically removing an old plate and mounting a new plate or a semi-automatic plate exchange apparatus for manually inserting a plate into gripper surfaces of a plate lockup device have been developed.
Of these plate exchange apparatuses, the semi-automatic plate exchange apparatus comprises a pair of track tables supported by an upper portion of a printing unit and suspending along right and left frames, right and left roller arms driven to be swingable and vertically movable along the track tables, and a plate press roller rotatably supported between free end portions of the roller arms, as disclosed in Japanese Patent Laid-Open No. 63-191636. The plate press roller is brought into contact with or separated from the circumferential surface of the plate cylinder upon swingable movement of the roller arms. A fixed plate guide is arranged at upper end portions of the track tables.
With the above structure, when a plate is to be mounted on a plate cylinder, the roller arms are moved downward to separate the plate press roller from the circumferential surface of the plate cylinder. The plate guided in contact with the plate guide is manually held to grip the plate with a leading-side plate lockup device, and the roller arms are swung to bring the plate press roller into contact with the plate. In this state, the plate cylinder is rotated by almost one revolution to wind the plate on the circumferential surface of the plate cylinder. At the same time, the roller arms are moved upward to stop rotation of the plate cylinder at a position where the plate press roller opposes a trailing-side plate lockup device. The trailing end of the plate is gripped by the trailing-side plate lockup device, thereby completely mounting the plate. That is, the plate press roller is operated during winding of the plate around the plate cylinder and gripping of the trailing end of the plate. The plate press roller is not operated during gripping of the plate on the leading side.
In this conventional plate exchange apparatus, however, when the plate is to be inserted into the gripper surfaces of the leading-side plate lockup device, the plate must be manually held as in the conventional case. It is not easy to insert the plate into a narrow opening. If the plate thickness is small, it is difficult for a single operator to hold the entire thin plate and perform this operation, thus disabling reduction in labor. In addition, when the trailing end of the plate is to be inserted into the gripper surfaces of the trailing-side plate lockup device under pressure, the plate may be undesirably bent. When the radial position of the plate press roller with respect to the plate cylinder is inappropriate upon bringing the plate press roller into contact with the circumferential surface of the plate press roller, the plate is gripped while an insertion amount of the tailing end of the plate into the gripper surfaces is insufficiently small. Alternatively, the trailing end of the plate is excessively pressed. In either case, the printing position is deviated from a proper position, thereby degrading the quality of printed matters.